Machines such as, for example, on and off-highway trucks, wheel loaders, motor graders, and other heavy equipment are used to perform many tasks. To effectively perform these tasks, the machines require an engine that provides significant torque through a transmission to one or more ground engaging devices. The transmission must provide a range of gearing in order to allow the machine to work at different speeds while keeping the engine operating within a desired operating range. For this purpose, the machines typically include a multi-speed, step-change, mechanical transmission connected between the engine and ground engaging devices.
To generate the wide range of gearing required by the machine, the mechanical transmission includes a plurality of intermeshing gears, a corresponding shift mechanism also commonly known as a shift yoke or rod, and/or one or more hydraulically actuated clutches and brakes. Each of the gears has a different number of teeth and the output gear ratio or speed of the transmission depends on the combination of engaged gears. The shift mechanism clutches, and/or brakes are used to selectively engage predetermined combinations of gears that result in a desired output ratio. To initiate intermittent movement of the shift mechanism and filling or draining of the clutches and brakes during a shifting operation, high pressure fluid at a high flow rate is required.
During operation of the transmission, the different components of the transmission require lubrication and cooling. Specifically, in order to reduce wear of the intermeshing gears, lubricant must be continuously supplied at low pressure to the interface of the gears and to other moving components within the transmission. This lubricant is then passed through a cooler to maintain a predetermined operating temperature within the transmission.
Historically, the intermittent high-pressure, high-flow, fluid demand and the continuous, low-pressure, lubricant and cooling demands of the step-change transmission were satisfied with a single fixed or variable displacement pump. The single fixed displacement pump was sized to supply the maximum pressure and flow demand at low engine speed and, because of its fixed displacement, was inefficient during times of low-pressure and low-flow demand, and/or during high engine speed situations. The single variable displacement pump was expensive and difficult to control. In addition, both the single fixed and single variable displacement pumps often cavitated under high-pressure demands.
One attempt to improve the efficiency of such a transmission is described in U.S. Pat. No. 5,513,732 (the '732 patent) issued to Goates on May 7, 1996. The '732 patent discloses an automatic transmission having a small, fixed displacement, high-pressure, oil pump and a larger, fixed displacement, low-pressure, oil pump. The output from the low-pressure pump provides fluid line pressure to assist the high-pressure pump and provides fluid line pressure to lubrication and cooler circuits. The high-pressure pump maintains fluid line pressure for operation of clutches, brakes, valves, and a torque converter. When the high pressure pump is capable of providing the required flow to the high-pressure system components (e.g., the clutches, brakes, valves, and torque converter), the pressure of the low-pressure pump is reduced by way of a relief valve to conserve power.
Although the automatic transmission of the '732 patent may improve efficiency, control, and stability over previous single pump transmissions, it may still lack optimum efficiency. Specifically, because the fixed displacement high-pressure pump of the '732 transmission maintains high fluid line pressure even when the demand for high pressure is low or non-existent, the automatic transmission may unnecessarily waste power. Any unnecessary waste of power translates to lost profits for the machine owner.
The disclosed transmission is directed to overcoming one or more of the problems set forth above.